1. Field of the Invention
The invention relates to a high temperature and oxidation resistant organopolysiloxane coating useful for repairing, bonding and for protecting porous thermal insulation from aeroconvective thermal degradation. More particularly, the invention relates to a room temperature curing, liquid organopolysiloxane polymer which contains unreacted silanol groups useful for repairing, bonding and for protecting porous ceramic insulation from aeroconvective thermal degradation and to the repaired, bonded or coated ceramic as a composite. On exposure to a high temperature environment, the coating forms an oxidation resistant, protective ceramic. The organopolysiloxane polymer is formed by the hydrolysis and partial condensation of di- and trifunctional organoalkoxysilanes with water.
2. Background of the Disclosure
Reentry space vehicles, such as the space shuttle which must leave and reenter the earth's atmosphere, require exterior thermal protection. The successful operation of the space shuttle required the development of light weight and very thermally efficient exterior thermal protection systems which have to withstand a wide variety of environments. These thermal protection systems (TPS) are in the form of rigid surface insulation in areas of high temperature (1000-1400.degree. C.), flexible blanket insulation in areas of moderate high temperature (below 1000.degree. C.) and oxidation protected, reinforced, rigid carbon/carbon in areas of severe temperatures (1400-1600.degree. C.). Thermal protection systems employ both flexible and rigid ceramic insulation, with the flexible insulation often comprising various layers fabricated of ceramic fibers and which may or may not include layers of metal foil. The space shuttle requires a light weight and very thermally efficient, rigid and flexible exterior TPS which has to withstand a wide variety of environments, including temperatures of from 1000-1600.degree. C. Ceramic materials currently in use or in development for such (TPS) include high purity silicon dioxide, aluminum oxide, silicon carbide, aluminosilicates, aluminoborosilicates and zirconium diboride as illustrative, but nonlimiting examples. In order to be both light weight and thermally protective, these ceramics and ceramic composites are very porous, often having a void volume of over 90%.
This porosity creates problems. One problem relates to hot gas penetration into the ceramic in the high temperature aeroconvective environment encountered on reentry. If this occurs, it can fuse the substrate and overheat the outer skin of the vehicle under the insulation. Another problem relates to embrittlement of the insulation in the high temperature aeroconvective environment encountered during reentry, which makes it more susceptible to damage. Attempts to resolve these problems have included very costly, time consuming, difficult to apply and easily damaged glassy coatings of the type disclosed in U.S. Pat. Nos. 4,093,771 and 5,296,288 which cannot be applied to, and cured on, insulation while it is on a vehicle. Low density, porous and lightweight rigid silica tiles and rigidized refractory composites prepared from silica fibers, along with aluminosilicate and borosilicate fibers and the like, as disclosed in U.S. Pat. No. 4,148,962, are used over much of the shuttle's surface. The light weight and concomitant low density of these tiles result from the porous structure. The outer surface of these tiles is sealed by applying a glassy coating of the type disclosed in U.S. Pat. No. 4,093,771 to the outer surface before they are attached to the space shuttle. This glassy coating has good resistance to devitrification and thermal shock, exhibits a low surface catalytic effect in a high temperature aeroconvective environment and can be made with either a high or low emittance. However, it is brittle and can be damaged by handling, hail, impact with space debris and the like. Attempts to resolve this problem have included costly, time consuming, difficult to apply and easily damaged glassy coatings of the type disclosed in U.S. Pat. No. 5,296,288 which has not yet met with much success. Therefore, a need exists for a better material to protect the thermal insulation and for a better method of repairing damage to both the glassy coating and to the insulation itself Therefore, there is still a need for a facile method and material for coating and also for sealing rigid porous insulation of a TPS against a high temperature aeroconvective environment.